Caught in the Act:
VLA Sees Star Turning Into Planetary Nebula

A team of astronomers using the National Science Foundation's
Very Large Array (VLA) radio telescope has caught an old star
during the very brief period of its transformation into a planetary
nebula, a shining bubble of glowing gas with a hot remnant star
at its center.

"This is the first time that anyone has seen a star that is so
clearly going through this transformation stage," said Yolanda Gomez,
an astronomer at the Institute for Astronomy at the National Autonomous
University in Mexico City, Mexico. "We believe this star began to
enter its planetary-nebula phase only after 1984," she added. The
researchers reported their findings in the November 15 edition of
the scientific journal Nature.

At the end of their lives, stars like our Sun eject gas into space
before starting to contract under their own gravity into white dwarf
stars. The gravitational contraction heats up the star, making it
pour out energetic ultraviolet light. The ultraviolet light tears
apart molecules in the gas ejected earlier by the star and rips
electrons from the atoms in the gas. This makes the gas glow,
producing often-beautiful shining shells and other shapes.

Once the remnant star has heated up sufficiently to produce large
amounts of ultraviolet light, molecules in the gas ejected earlier
are destroyed rapidly. "We are seeing radio waves emitted by water
molecules in this planetary nebula," said Gomez, who added, "The
water molecules, we believe, are all destroyed within only 100 years
of the beginning of this stage, so we are seeing this star during
an extremely brief transition period of its life." This is the first
time that water has been detected in a planetary nebula.

The astronomers used the VLA to observe a planetary nebula called
K3-35, 16,000 light-years from Earth in the constellation Vulpecula
(the small fox). This object has a doughnut-shaped ring of gas around
its center and lobes of outflowing material, similar to structures
seen in other planetary nebulae.

The researchers were surprised to find regions near the star in which
water molecules are amplifying, or strengthening radio-wave emission
at a frequency of 22 GigaHertz, in the same manner that a laser
amplifies light waves. They found these regions, called masers, in
the doughnut-shaped structure surrounding the central star, as well
as at the end of much larger lobes of gas extending from the star.
The doughnut-shaped ring has a radius of more than twice the distance
from the Sun to Pluto. The masers at the ends of the lobes are more
than 100 times more distant from the star.

By analyzing their VLA observations as well as earlier observations
of the object by other astronomers, the research team concludes that
K3-35 has only just begun its transformation into a planetary nebula.

"This is extremely exciting, because we now have a 'laboratory' for
watching this process take place over the next few years," Luis F. Miranda
of the Astrophysical Institute of Andalucia (IAA) in Granada, Spain said.
"We don't fully understand
everything we see in this object, but know that we are going to learn
much valuable information about this process by watching it develop,"
he added.

"We are very lucky to have caught this star during such a very brief
but important period of its life," agreed Guillem Anglada of the IAA and the
Harvard- Smithsonian Center for Astrophysics in Cambridge, MA and
Jose Torrelles of the Institute for Space Studies of Catalunya in
Barcelona, Spain, who are, with Miranda and Gomez, members of the
research team.